Scleranthus annuus L., Annual Knawel, is a widely accepted species, but its limits are constantly under review. Stace (1997) includes within it S. polycarpos L., as S. annuus L. ssp. polycarpos Bonnier & Layens. It is an annual or biennial plant with inconspicuous green flowers. The size of the achene and the shape of the flower distinguish the two subspecies.
Chromosome No.: 2n = 44 (Stace 2010).
Photography: G. Toone
It is widespread thoughout Britain, but absent from the very north and the Outer Hebrides. In Ireland it is rare but scattered throughout.
The ssp. polycarpos is recorded mainly in the Breckland, although there are a few records from other parts of the country. Lusby (in Preston et al. 2002) suggests that it is under-recorded.
The species occurs throughout Europe except in the extreme north and is introduced in North and South America, Australia and doubtless elsewhere (Tutin et al. 1993; GBIF)
Its maximum recorded altitude is given by Dickie (1860) as 1200ft (365 m) in Aberdeenshire, although the precise locality is not known.
Photography: G. Toone
It tends to occur in two distinct habitats - arable fields and dry, heathy grassland. The latter would be more obviously a natural habitat for it, and Hanbury & Marshall (op. cit.), for instance, considered plants in such situations (which they called S. biennis) to be ‘the original form’, whereas ‘the commoner plant [is] a more luxuriant product of cultivated land.’ Rodwell (1991-2000) lists it only as a plant of arable fields in the SE of England, in OV1 Viola arvensis-Aphanes microcarpa and OV5 Digitaria ischaemum-Erodium cicutarium communities.
- Origin: native. The first record is considered to have been by Thomas Johnson in 1629, at Quex in Kent (Hanbury & Marshall 1899), although Clarke (1900) points out that there may have been some confusion between this species and Alchemilla arvensis.
- Rarity: rare in Ireland, but not in Britain.
- Threat: it has recently been added to the British Red List as ‘Endangered’ owing to an apparently significant decline (Cheffings & Farrell 2005). In Ireland, the decline seems even more severe.
- Conservation: most counties count it as an axiophyte, either of species-rich arable fields or of heaths and upland grassland.
It is not entirely apparent what is happening to Scleranthus annuus. Its range has not contracted, but it has seemingly declined from its arable situations. Whether the more natural populations have changed or not is an unanswered question. There is need for more study of the putative subspecies, including cultivation experiments to demonstrate that they breed true and possibly genetics work to see what taxonomic rank they merit. More work is needed on its vegetation communities, especially in non-arable habitats.
- Dickie, G. 1860. The Botanist’s Guide to the Counties of Aberdeen, Banff and Kincardine. Brown, Aberdeen.
- Hanbury, F.J. & Marshall, F.J. 1899. Flora of Kent. Frederick J. Hanbury, London.